The present invention relates to a wafer chuck used in manufacturing processes of integrated semiconductors and liquid crystal panels and, more particularly, to an electrostatic silicone rubber chuck for ion injectors used in an ion injection process.
In the ion injection process for manufacturing integrated semiconductors or liquid crystal panels, the so-called mechanical chuck utilizing clamps and the so-called electrostatic chuck, such as a wafer chuck utilizing electrostatic adsorption or Johnsen-Rahbeck force, have so far been employed. When the mechanical chuck is in operation, clamps mechanically press down on a wafer and thereby the wafer is warped. In addition, no devices can be formed on the areas where the wafer is covered with the clamps, and so effective wafer space is reduced. With the intention of obviating those defects, therefore, electrostatic chucks have been proposed, and they are operational at present. Examples of an insulating layer with which those electrostatic chucks are provided include layers of plastics such as polyimide, those of ceramics such as alumina and aluminum nitride, and those of gum elastic solids such as silicone rubber.
In the ion injection process, on the other hand, it is required to inhibit wafers from suffering a temperature rise due to the heat evolved by ion beam injection and keep the wafer temperature uniform and constant, thereby ensuring consistent ion injection without thermal damage to the wafers. In order to meet such a requirement, platen apparatus for cooling wafers is in practical use. For instance, such an apparatus is equipped with a cooling mechanism of passing a chiller through channels formed on the back of electrostatic chucks and in the interior of a mount.
In the case of electrostatic chucks constructed from ceramics, their insulating layers to be brought into contact with wafers are so hard that they have inferior conformability to asperity on the back of each wafer. As a result, the thermal resistance between mating surfaces becomes great, and satisfactory heat-dissipating characteristics cannot be achieved. In order to dissolve this problem, the method of passing an inert gas flow, such as a helium flow, through the gap between a wafer and an insulating layer has been proposed, and put to practical use. Therein, the gas flow is used as an intermediate for thermal transfer between the wafer and the insulating layer. However, such a method requires not only micro-machining for forming inert gas flow-passing grooves on the insulating layer surface but also a setup for feeding an inert gas flow, and thereby a rise in production cost is caused.
On the other hand, the electrostatic chucks constructed from polyimides are prevailingly used at present because they can be manufactured with ease and at low prices. However, their thermal conductivity is low and their hardness is high. Therefore, as in the case of electrostatic chucks constructed from ceramics, the polyimide electrostatic chucks have a drawback of being insufficient in heat-dissipating characteristics because their poor conformability to asperity on the back of a wafer causes high thermal resistance between the mating surfaces.
In comparison with the aforementioned chucks, electrostatic chucks constructed from silicone rubbers (as disclosed in Japanese Tokko Hei 2-55175 and Hei 2-63307, wherein the term xe2x80x9cTokkoxe2x80x9d means an xe2x80x9cexamined patent publicationxe2x80x9d, and Japanese Tokkai Hei 9-298233, wherein the term xe2x80x9cTokkaixe2x80x9d means an xe2x80x9cunexamined published patent applicationxe2x80x9d) have good conformability to asperity on the back of a wafer because elastic silicone rubber is used for their insulating layers. In the case of chucks utilizing highly heat-conductive silicone rubber in particular, the wafer temperature can be kept uniform with high efficiency. Additionally, general electrostatic chucks constructed from silicone rubber have a structure that a pattern formed of metal foil traces to function as an internal electrode is sandwiched between two sheets of thermally conductive silicone rubber.
In manufacturing a thermally conductive silicone rubber sheet used in such a case, a preform is made first from a thermally conductive silicone rubber composition. Therein, the rubber composition is generally constituted of organopolysiloxane, an inorganic powder having high thermal conductivity, such as boron nitride or aluminum oxide, and a curing agent. More specifically, the preform is made by sheeting the composition on a plastic film by the use of a calender technique, or by dispersing the composition into an organic solvent, shaping the composition into a sheet on a plastic film or glass cloth and then drying it. Then, the preform thus made is subjected to press vulcanization.
In the cases of hitherto known thermally conductive silicone rubber compositions, however, increasing the inorganic powder contents therein with the intention of enhancing their thermal conductivities gives rise to a reduction in the rubber strength. As a result, it becomes difficult to peel silicone rubber sheets apart from a mold or a plastic film. In order to improve the release capability, it has so far been carried out to add an internal release agent, such as zinc stearate, to a thermally conductive silicone rubber composition. Therefore, the use of such a thermally conductive silicone rubber for an electrostatic chuck causes a problem of contaminating silicone wafers with zinc of zinc stearate origin.
In recent years, electrostatic chucks have been upsized to the order of 300-400 mm in diameter to keep pace with the increase in diameter of wafers. In the case of liquid crystal panels, further upsizing is required for electrostatic chucks because those panels have come to use a substrate having a size of 1,000 mm per side. In forming a thermally conductive silicone rubber into such large-sized sheets, it is difficult to release the sheets from molds or the like even when the rubber contains a known internal release agent, such as zinc stearate. Under these circumstances, it has been expected to develop an internal release agent capable of imparting improved release capability to a thermally conductive silicone rubber composition in its molding process, and besides, causing no contamination of wafers by metals.
Therefore, an object of the invention is to provide an electrostatic silicone rubber chuck for ion injection that has high releasability from a mold and causes no contamination of wafers.
The present object as described above is attained with an electrostatic chuck for ion injectors, comprising a metallic substrate, a first insulating layer, a pattern of conductive trace or traces formed as a single or dual electrode on the first insulating and a second insulating layer provided on the conductive trace or traces: with at least the second insulating layer being produced from cured matter of a thermally conductive silicone rubber composition comprising the following components (A) to (D);
(A) 10 to 69.99 volume % of organopolysiloxanes having an average compositional formula R1aSiO(4xe2x88x92a)/2, wherein R1 groups are the same or different unsubstituted or substituted monovalent hydrocarbon groups and a is a positive number of from 1.90 to 2.05,
(B) 30 to 89.99 volume % of a thermally conductive filler,
(C) 0.01 to 10 volume % of a fluorine-modified silicone surfactant, and
(D) a curing agent in an amount required for curing the composition containing the components (A), (B) and (C),
wherein the total volume % of the components (A), (B) and (C) is adjusted to 100.